Method of controlling bulk density of ferrous powder

ABSTRACT

The bulk density of ferrous powder made by atomizing molten steel is controlled by adding oxygen to the atomizing process and by reducing subsequently any resulting oxides to obtain a spongy surface layer of the particles, having thickness in dependence upon the added oxygen and reducing bulk density accordingly.

United States Patent 1 Findeisen et al.

[ Sept. 30, 1975 1 METHOD OF CONTROLLING BULK DENSITY OF F ERROUS POWDER [75] Inventors: Gerhard Findeisen, Rheydt; Norbert Dautzenberg, Meerbusch; Hartmut Gesell, Duisburg, all of Germany [73] Assignee: Mannesmann Aktiengesellschaft,

Dusseldorf, Germany [22} Filed: June 27, 1973 [2]] Appl. No.: 374,100

[30] Foreign Application Priority Data June 29, 1972 Germany 2232760 [52] U.S. Cl. 75/.5 BA; 75/.5 C; 264/12 [51] Int. Cl. B22F 9/00 [58] Field of Search 75/.5 BA, .5 C; 264/11,

[56] References Cited UNITED STATES PATENTS 2,159,433 5/1939 Ervin 264/111 X 2.341.704 2/1944 I Ervin 75/.5 C 2,384,892 9/1945 Comstock 1 75/.5 C 3,353,951 11/1967 Shaferet al. 75/.5 BA 3,764,295 10/1973 Lindskog 75/.5 C X Primary Examiner--L. Dewayne Rutledge Assistant Examiner-Arthur J Steiner Attorney, Agent, or FirmRalf H. Siegemund 5 Claims, N0 Drawings METHOD OF CONTROLLING BULK DENSITY OlF FERROUS POWDER BACKGROUND OF THE INVENTION The present invention relates to a method of making ferrous powder of variable bulk density.

Iron powder is used as raw material for making various components, for example, by compressing such powder in a mold. The powder used for that purpose is usually unalloyed, i.e., more or less'pure iron, and such powder is made in accordance with a variety of processes. Of particular importance here are methods of directly reducing iron ore or scale resulting from rolling (powder of spongy iron). In accordance with other known methods liquidous iron is atomized by air jets or by water jets. Direct reduction and atomization processes are combined in a method wherein iron powder, as produced by atomization, and having a relatively high content of carbon, is mixed with oxygen carriers (iron and/or rolling scale) and annealed i.e. malleableized.

Iron powder as made in accordance with the several known processes differ very little in chemical composition as well as in the size distribution of the particles. However the geometry of the particles varies significantly with the method of producing them. Consequently, the bulk densities of the different powders differ due to these differences in particle configuration. The resulting bulk density is, therefore, to some extent characteristic of the method by which the particular powder was made. For example, particles made by directly reducing iron ore or made by air jet atomization have hollow spherical configuration i.e. they are spongy, and in bulk they establish a ,light powder. Powder made by waterjet atomization consists of parti cles which are compact with an irregular, spattered surface. Such a powder is relatively heavy. Powder made by the combination process is of medium weight accordingly.

By way of example, light powders have bulk density of 2.5 to 2.6 grams per cubic centimeter. The heavy powders have bulk density of 3.1 to 3.3 grams per cubic centimeter and the combination method produces powder in the range of 2.7 to 2.9 grams per cubic centi meter. i

It will be appreciated that upon pressing iron powder into a mold or a die the bulk density of the powder is a significant parameter to be considered for the design of the tool, particularly because the tools have to be dimensioned accordingly. The presently used powder pressing methods use metering of the powder as to volume. Metering by weight is possible but more compli cated.

Powder press tools and machines are usually constructed so that parts of different weights and different dimensions can be pressed with the same machine. The volume of filling the die cavity depends on the cavity dimensions in the die plate proper. The position of the lower punch determines the volume of the cavity, and the bulk density of the powder is an additional parameter. Volume metering of the powder is critical for obtaining the predetermined weight or density of the product as resulting from compressing. It is necessary, therefore, to maintain the bulk density of the powder within very close tolerances.

It follows from the foregoing that the construction of the pressing tool generally depends to a considerable extent on the range of the bulk density of the available powder. An exchange of powder grades with different apparent densities is hardly possible, at least not for many uses. If the die is operated in fixed steps, such as for pressing sleeves or collars, it is actually impossible tovary the density of the powder used.

SUMMARY OF THE INVENTION It is an object of the present invention to suggest a method for making iron powder of various bulk densities, possibly covering all of the above mentioned ranges, but using one method under control of a process parameter.

It is another object of the present invention to make ferrous powder by means of a single type atomization process but resulting in powder that has particular density within the range from below 2.5 g/cm to above 3.3 g/cm It is, therefore, an object of the present invention to make ferrous powder by means of a particular atomization process in which quantitatively a process parameter can befvaried to thereby vary the resulting bulk density of the powder.

In accordance with the preferr'ed'embodiment of the invention it is suggested to pour liquidous iron or steel and to atomize the liquid metal stream by means of waterjets as ejected from a nozzle or nozzles and to provide thereto additional oxygen, either through additive gaseous oxygen or by producing oxygen at the nozzle, so that the partial pressure of the oxygen so added is larger than the partial pressure of any inherent oxygen in the water as well as in the outer atmosphere. The supply or development of that additionaloxygen is to be made higher, the lower a bulk density one wants to obtain for the powder. The powder is subsequently reduced for removing the oxygen skin on the individual powder particles as formed through the addition of oxygen to the process.

DETAILED DESCRIPTION OF THE INVENTION For practicing the invention, basically a conventional atomizer can be used, wherein a stream of molten iron or steel pours from a bottom opening of a ladle and passes a nozzle or nozzles directing pressurized jets of water against the down pouring metal stream. The metal stream as such disintegrates and multiple droplets coagulate, falling to the bottom while solidifying, at least at their surface before piling on the bottom. The atomizing water is provided under pressure to a chamber with a nozzle or nozzles from which the water jets are ejected. Now, in accordance with the invention it is suggested to feed pure oxygen or oxygen enriched air to that chamber so that a water-oxygen mixture be ejected through the nozzle or nozzles. Additionally or alternatively, the pressurized water is mixed with hydrogen peroxide, which gives off the oxygen at the point of atomization.

As a consequence, a raw powder is produced with differently strong an-oxidation at the surface of the individual particles. The subsequent reduction converts, so to speak, this oxidation into a spongy coating on and as part of the surface of the particles. That coating is the thicker, the thicker an an-oxidation layer has formed on the particles prior to reduction, and that thickness, in turn, is the greater the more oxygen was added and/or developed. The apparent density and bulk density of the final powder decreases with increasing dimension of that spongy coating so that accordingly the bulk density increases with decreasing anoxidation.

It follows that the amount of oxygen added under controlled conditions to the water jet atomizing process controls the resulting bulk density, and variations here permit production of iron powder with bulk density as low as 2.5 g/cm or even lower or as high as 3.2 g/cm". The oxygen added under controlled conditions has a higher pressure than the partial pressure of oxygen in water as occurring normally. The added oxygen may have a pressure of 2 to 6 atmospheres.

It is uneconomical to reduce the raw powder by means of adding, e.g., hydrogen. Rather, one will use internal reaction. For example, molten steel to be atomized as enriched with carbon to such an extent that the carbon and the added oxygen have stoichiometrical relation for formation of carbon dioxide during malleableizing in its own atmosphere.

Turning now to some specific examples, the result of normal water jet atomization will be explained first, and the augmentation of that process by the invention will be explained thereafter.

The atomizer employed may be an annular nozzle such as shown, e.g. in U.S. Pat. No. 2,892,215, or patent applications of common assignee Ser. No. 251,839, filed May 9, 1972 or Ser. No. 227,044, filed Feb. 17, 1972. Molten steel pours through a central opening of such a nozzle chamber which directs jets of pressurized water against the stream from all sides. The molten steel was composed of iron with 0.1% C, 0.10% Si and 0.20% Mn. Assuming first a conventional process, just water was applied under pressure of about 50 atmospheres, for atomizing the steel and forming particles of not more than 0.4 mm diameter. An inherent an-oxidation of the particles produced an oxygen content of about 1% which was reduced by N11 to a level of 0.2%. The bulk density of the final product was about 3.2 g/cm Another process, using the same type of equipment but processing carbonized steel, i.e. steel having 1.5% C, lead to an-oxidation because of the use of water, and the resulting powder had a content of 1.4 C and 0.8% 0. After reduction in an NH; atmosphere an unusable powder with 1.1 C and 0.10% O was obtained. The bulk density was also about 3.2 g/cm.

In accordance with the invention, the process was supplemented by supplying oxygen to the nozzle chamber. As stated, the water was supplied under pressure of 50 atmosphere. The additional oxygen was applied at a pressure of a few atmospheres. By means of injector effect, air was sucked into the nozzle so that a water and oxygen-enriched-air-mixture provided the atomization. The basic atomizing agent was, of course, water, but the added oxygen controlled the resulting powder density. The addition of oxygen can be controlled either through pressure control or by adjusting the effective cross section of the flow path for the oxygen.

In the case of a carbonized steel melt with 1.5% C, a powder with a carbon content of 1.3% was obtained, but the oxygen content of the powder was 2.3%. After annealing-malleableizing at 1050 C, the carbon content in the powder was reduced to 0.5% and the oxygen content was reduced to 0.3%. The bulk density of the powder was 2.6 g/cm. The particles had a spongy surface texture.

In accordance with a different example of the invention, bulk density can be controlled by adding hydrogen peroxide to the atomizing water. One will use here 0.1 to 20% to cover more than the stated range. Preferably 0.2% to 4% peroxide is added to the atomizing water for purposes of density control. The oxygen is developed at the area of steel-atomizing agent interaction.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

We claim:

1. In a method for atomizing molten iron or steel by means of pressurized water atomizing a pouring stream of the liquidous iron or steel to obtain a ferrous powder which is subsequently annealed for reducing the oxygen and carbon content and which is to be used for press-forming of parts by pressing such powder into a mold, the improvement comprising, adding pure oxygen or oxygen enriched air at an oxygen pressure from 2 to 6 atmospheres to the water to enrich the relative oxygen content in the water, so that the partial pressure of the oxygen is larger than normal; and using the oxygen-enriched water for the atomization so as to increase the oxidation layer on the powder particles as produced by the atomization beyond the layer thickness as resulting from mere use of water as atomizing agent, so that the surface of the individual powder particles are spongy as resulting from removal of the oxygen during subsequent annealing to obtain a reduction in the bulk density of the powder.

2. Method as in claim 1, wherein the molten steel is atomized by water at a pressure of about atmospheres.

3. In a method for atomizing molten iron or steel by means of pressurized water atomizing a pouring stream of the liquidous iron or steel to obtain a ferrous powder which is subsequently annealed for reducing the oxygen and carbon content and which is to be used for press-forming of parts by pressing such powder into a mold, the improvement comprising, adding about 0.1 to 20 hydrogen peroxide to the water to enrich the relative oxygen content inthe water, the content being the higher, the lower a bulk density of the powder to be made is desired; and using the oxygen-enriched water for the atomization so as to increase the oxidation layer on the powder particles as produced by the atomization beyond the layer thickness as resulting from mere use of water as atomizing agent, so that the surface of the individual powder particles are spongy as resulting from removal of the oxygen during subsequent annealing to obtain a reduction in the bulk density of the powder.

4. In a method as in claim 3, wherein the amount of hydrogen peroxide added is about 0.2 to 4 5. In a method for atomizing molten iron or steel by means pressurized water atomizing a pouring stream of the liquidous iron or steel to obtain a ferrous powder which is subsequently annealed for reducing the oxygen and carbon content and which is to be used for p'ressfortning of parts by pressing such powder into a mold, the improvement comprising, using water for atomization which is enriched with gasesous oxygen at a partial pressure in excess of the partial pressure of oxygen in regular water, so as to increase the oxidation layer on the powder particles as produced by the atomization beyond the layer thickness as resulting from mere use of water as atomizing agent, so that the surface of the individual powder particles are spongy as resulting from removal of the oxygen during subsequent annealing to obtain a reduction in the bulk density of the powder. 

1. IN A METHOD FOR ATOMIZING MOLTEN IRON OR STEEL BY MEANS OF PRESSURIZED WATER ATOMIZING A POURING STREAM OF THE LIQUIDOUS IRON OR STEEL TO OBTAIN A FERROUS POWDER WHICH IS SUBSEQUENTLY ANNEALED FOR REDUCING THE OXYGEN AND CARBON CONTENT AND WHICH IS TO BE USED FOR PRESS-FORMING OF PARTS BY PRESSING SUCH POWDER INTO A MOLD THE IMPROVEMENT COMPRISING, ADDING PURE OXYGEN OR OXYGEN ENRICHED AIR AT AN OXYGEN PRESSURE FROM 2 TO 6 ATMOSPHERES TO THE WATER TO ENRICH THE RELATIVE OXYGEN CONTENT IN THE WATER, SO THAT THE PARTIAL PRESSURE OF THE OXYGEN IS LARGER THAN NORMAL, AND USING THE OXYGEN-ENRICHED WATER FOR THE ATOMIZATION SO AS TO INCREASE THE OXIDATION LAYER ON THE POWDER PARTICLES AS PRODUCED BY THE ATOMIZATION BEYOND THE LAYER THICKNESS AS RESULTING FROM MERE USE OF WATER AS ATOMIZING AGENT, SO THAT THE SURFACE OF THE INDIVIDUAL POWDER PARTICLES ARE SPONGY AS RESULTING FROM REMOVAL OF THE OXYGEN DURING SUBSEQUENT ANNEALING TO OBTAIN A REDUCTION IN THE BULK DENSITY OF THE POWDER.
 2. Method as in claim 1, wherein the molten steel is atomized by water at a pressure of about 50 atmospheres.
 3. In a method for atomizing molten iron or steel by means of pressurized water atomizing a pouring stream of the liquidous iron or steel to obtain a ferrous powder which is subsequently annealed for reducing the oxygen and carbon content and which is to be used for press-forming of parts by pressing such powder into a mold, the improvement comprising, adding about 0.1 to 20 % hydrogen peroxide to the water to enrich the relative oxygen content inthe water, the content being the higher, the lower a bulk density of the powder to be made is desired; and using the oxygen-enriched water for the atomization so as to increase the oxidation layer on the powder particles as produced by the atomization beyond the layer thickness as resulting from mere use of water as atomizing agent, so that the surface of the individual powder particles are spongy as resulting from removal of the oxygen during subsequent annealing to obtain a reduction in the bulk density of the powder.
 4. In a method as in claim 3, wherein the amount of hydrogen peroxide added is about 0.2 % to 4 %.
 5. In a method for atomizing molten iron or steel by means pressurized water atomizing a pouring stream of the liquidous iron or steel to obtain a ferrous powder which is subsequently annealed for reducing the oxygen and carbon content and which is to be used for pressforming of parts by pressing such powder into a mold, the improvement comprising, using water for atomization which is enriched with gasesous oxygen at a partial pressure in excess of the partial pressure of oxygen in regular water, so as to increase the oxidation layer on the powder particles as produced by the atomization beyond the layer thickness as resulting from mere use of water as atomizing agent, so that the surface of the individual powder particles are spongy as resulting from removeal of the oxygen during subsequent annealing to obtain a reduction in the bulk density of the powder. 